Special Situations in the Simplex Algorithm


 Randall Hampton
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1 Special Situations in the Simplex Algorithm Degeneracy Consider the linear program: Maximize 2x 1 +x 2 Subject to: 4x 1 +3x 2 12 (1) 4x 1 +x 2 8 (2) 4x 1 +2x 2 8 (3) x 1, x 2 0. We will first apply the Simplex algorithm to this problem. After a couple of iterations, we will hit a degenerate solution, which is why this example is chosen. We will then examine the geometrical origin of degeneracy and the related issue of cycling in the Simplex algorithm, with the help of the graphical representation of this problem. After introducing three slack variables and setting up the objective function, we obtain the following initial Simplex tableau. s 3 Variable s s s With x 1 as the entering variable, there is a tie for the minimum ratio, at R 2 and R 3. This (also observed in the previous twophase example) implies that after a pivot with either R 2 or R 3 as the pivot row, the resulting tableau will have a degenerate basic variable. Let us choose R 2 (say) as the pivot row. Then, after executing a pivot, we obtain the tableau below. Tableau I: s 3 Variable 1 0 1/2 0 1/2 0 4 s x /4 0 1/4 0 2 s The current basic feasible solution is (x 1, x 2, s 1, s 2, s 3 ) = (2, 0, 4, 0, 0), where s 3 is (as expected) a degenerate basic variable. The next pivot column and pivot row will be the x 2 column and R 3, respectively. After executing another pivot, we obtain the following 1
2 tableau. Tableau II: s 3 Variable /2 4 s x /2 1/4 2 x Again, the current basic feasible solution is (x 1, x 2, s 1, s 2, s 3 ) = (2, 0, 4, 0, 0). However, the identify of the degenerate basic variable has switched from s 3 to x 2. Note that this tableau happens to be optimal (independent of the phenomenon of degeneracy). To understand what it means to have a degenerate solution, let us now refer to the graphical representation of this problem, which is shown in Figure LP8. Notice that three, not two, constraint equations pass through the cornerpoint solution (x 1, x 2 ) = (2, 0). These equations are: x 2 = 0, 4x 1 + x 2 = 8, and 4x 1 + 2x 2 = 8. Since only two lines are needed to define such an intersection, we see that degeneracy is a manifestation of redundancy in information. That is, we can choose to let any pair of these equations (out of ( ) 3 = 3 2 combinations) to define this intersection. For example, if we choose x 2 = 0 and 4x 1 +x 2 = 8 as the defining equations, then, since the solution to this pair of equations will automatically satisfy equation 4x 1 + 2x 2 = 8, the value of the slack variable associated with the inequality 4x 1 + 2x 2 8, namely s 3, must turn out to be 0. This accounts for the appearance of the degenerate basic variable s 3 in Tableau I. Similarly, if we choose 4x 1 + x 2 = 8 and 4x 1 + 2x 2 = 8 as the defining equations, then the inequality constraint x 2 0 will turn out to be binding. This accounts for the fact that x 2 is a degenerate basic variable in Tableau II. What will happen if we choose x 2 = 0 and 4x 1 + 2x 2 = 8 as the defining equations? A careful examination of Tableau II shows that if we choose the s 2 column and R 3 as the pivot column and the pivot row, then the following tableau results after a pivot. Tableau III: s 3 Variable /2 4 s x / /4 2 s This new tableau, again, corresponds to the solution (x 1, x 2, s 1, s 2, s 3 ) = (2, 0, 4, 0, 0). Notice however that the slack variable s 2 associated with the inequality 4x 1 + x 2 8 has indeed replaced x 2 as the degenerate basic variable. 2
3 Recall that in the last pivot, the pivot element is a negative number, 1. This will never occur during ordinary Simplex iterations. (Why?) Our purpose for carrying out such a pivot is to show that there is indeed a third tableau that is associated with the cornerpoint solution (2, 0). Now, with three tableaus all corresponding to the same set of coordinates, the question is: Is it possible for the Simplex algorithm to cycle through these (or a subset of these) tableaus forever? Theoretically, the answer is yes. However, this happens rarely in practice, and can in fact be avoided. Observe that in order for the Simplex algorithm to cycle, we must repeat ourselves during the iterations. Suppose a (nonoptimal) tableau that has been visited before is generated as the result of a pivot. The idea is to look for a different choice for either the pivot column or the pivot row. Whenever such a choice can be found, we simply continue the algorithm with the new choice. It turns out that repeatedly doing this will eventually take us out of any degenerate solution. We will leave out the difficult theoretical argument that supports this last assertion. In terms of the mechanics of the Simplex algorithm, how does one get out of degeneracy? We will answer this with an example. Consider a tableau that has the configuration below. Pivot Column RHS a d b 0 Degeneracy c e Pivot Row Here, we assume that: we are maximizing, the column explicitly shown on the left is the pivot column, the entry a is negative, the entry b is nonpositive, the entry c is positive, the column shown on the right is the righthandside column, the entry e is positive, and finally the row containing c and e is the pivot row. Notice that in the row just above the pivot row, the righthandside constant equals 0, which indicates that the current solution is degenerate. However, since b is assumed to be nonpositive, we do not compute a ratio for this row in the ratio test. (To simplify discussion, we also assume that this is the only row with a zero on the righthand side.) This makes it possible for a row with a positive righthandside constant to become the pivot row. Now, when a pivot is performed with the entry c as the pivot element, we will multiply the pivot row by a/c and add the outcome into R 0. This will result in a strict improvement in the objectivefunction value, from d to d + ( a/c)e. With this strict increase, we are now guaranteed never to return to this tableau again in the remainder of the Simplex algorithm. 3
4 In summary, the phenomenon of cycling in the Simplex algorithm is caused by degeneracy. While cycling can be avoided, the presence of degenerate solutions may temporarily suspend progress in the algorithm. Unboundedness Consider the linear program: Maximize 2x 1 +x 2 Subject to: x 1 x 2 10 (1) 2x 1 x 2 40 (2) x 1, x 2 0. Again, we will first apply the Simplex algorithm to this problem. The algorithm will take us to a tableau that indicates unboundedness of the problem. We will then examine the geometrical origin of unboundedness with the help of the graphical representation of this problem. After introducing two slack variables and setting up the objective function, we obtain the following initial Simplex tableau. Variable s s With x 1 as the entering variable, it is easily seen that R 1 is the pivot row. After executing a pivot, we obtain the tableau below. Variable x s Since x 2 has a negative coefficient in R 0, this tableau is not optimal. Another pivot takes us to the next tableau. Variable x x This tableau again is not optimal. However, at this point, we are unable to perform further iterations, because as we attempt to carry out a ratio test with s 1 as the entering variable, 4
5 it turns out that there is no ratio to compute. What this means is that as we attempt to bring s 1 in as a basic variable, none of the constraints will stop us from increasing its value to infinity. Now, as the value of s 1 increases, the objectivefunction value will also increase correspondingly at a rate of 4. It follows that the problem does not have an optimal solution. The feasible region of this problem is depicted in Figure LP9. There, we see that the Simplex algorithm starts with the point (0, 0), follows the x 1 axis to the point (10, 0), rises diagonally to the point (30, 20), and then takes off to infinity along an infinite ray that emanates from (30, 20). More formally, what we have is that for any nonnegative δ, the solution (x 1, x 2, s 1, s 2 ) = (30 + δ, δ, δ, 0) is feasible. Since this solution has a corresponding objectivefunction value of δ, we see that the problem is unbounded. Clearly, unboundedness of a problem can occur only when the feasible region is unbounded, which, unfortunately, is something we cannot tell in advance of the solution attempt. In the above example, we detected unboundedness when we encountered a pivot column that does not contain any positive entry. More generally, we can in fact declare a problem as unbounded if any (nonbasic) column, not necessarily associated with the entering variable, is identified to have the abovestated property at the end of an iteration. Referring back to the initial tableau, we see that, indeed, the x 2 column had this property. Therefore, we could have concluded that the problem is unbounded at the outset. The difference is that the algorithm would then follow the x 2 axis to infinity. (Of course, another difference is the amount of effort.) The corresponding condition for unboundedness in a minimization problem is slightly different: We should look for a nonbasic column with a positive coefficient in R 0 and with all other entries nonpositive. In most applications of linear programming, if a problem turns out to be unbounded, it is often due to the fact that at least one relevant constraint has been left out during the formulation stage. Therefore, one should carefully reexamine the original formulation. Multiple Optimal Solutions Consider the linear program: Maximize 4x 1 +14x 2 Subject to: 2x 1 +7x 2 21 (1) 7x 1 +2x 2 21 (2) x 1, x
6 As before, we will first apply the Simplex algorithm to this problem. The algorithm will take us to a tableau that indicates that alternative optimal solutions exist. We will then examine the geometrical origin behind the existence of alternative optimal solutions, with the help of the graphical representation of this problem. After introducing two slack variables and setting up the objective function, we obtain the following initial Simplex tableau. Variable s s With x 2 as the entering variable, it is easily seen that R 1 is the pivot row. After executing a pivot, we obtain the tableau below. Variable x 2 0 2/7 1 1/7 0 3 s /7 0 2/ At this point, since every nonbasic variable has a nonnegative coefficient in R 0, the current solution (x 1, x 2, s 1, s 2 ) = (0, 3, 0, 15) is optimal. However, notice that the nonbasic variable x 1 has a coefficient of 0 in R 0. This implies that if we attempt to let x 1 enter the basis, then the objectivefunction value will not change. Indeed, after a pivot with the x 1 column as the pivot column, we obtain the following new tableau. Variable x /45 2/45 7/3 x /45 7/45 7/3 With the same objectivefunction value, the new solution (x 1, x 2, s 1, s 2 ) = (7/3, 7/3, 0, 0) is, of course, also optimal. Note that a further attempt at a pivot in the s 2 column will take us back to the previous solution. We will therefore not pursue things further. The feasible region of this problem is depicted in Figure LP10. There, we see that the Simplex algorithm starts with the point (0, 0), travels along the x 2 axis to the first optimal solution at (0, 3), and then continues on to the second optimal solution at (7/3, 7/3). Notice that the objectivefunction line 4x x 2 = c (for any c) is parallel to the edge that begins at (0, 3) and ends at (7/3, 7/3). Hence, every point on this edge is optimal. In general, if we are given two optimal solutions to a linear program, then an infinite number of optimal solutions can be constructed. In this example, both (0, 3) and (7/3, 7/3) are 6
7 optimal. Therefore, every point on the edge connecting these two points will also be optimal. Formally, points on this edge are traced out by solutions of the form: (x 1, x 2 ) = δ (0, 3) + (1 δ) (7/3, 7/3), where δ is any value in the interval [0, 1]. As specific examples, if we let δ = 1, then we have the point (0, 3); if we let δ = 0, then we have the point (7/3, 7/3); and if we let δ = 1/2, then we have the point (7/6, 8/3), which is half way between (0, 3) and (7/3, 7/3). 7
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